Overview of Substation

8/3/2019 Overview of Substation

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THE ASSEMBLY OF APPARATUS USED TO CHANGE SOMECHARACTERISTICS (I.E. VOLTAGE A.C TO D.C., FREQUENCY, POWER

FACTOR) OF ELECTRIC SUPPLY IS CALLED A SUBSTATION

SUBSTATION

GENERATING TRANSMISSION DISTRIBUTION


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A Transformer is a static piece of apparatus by

means of which electric power in one circuit is

transformed into electric power of the same frequencyin another circuit.


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Circuit breakers are mechanical devices

designed to close or open contact members, thus

closing or opening of an electrical circuit under

normal and abnormal conditions.

There are two methods of arc extinction in circuit breakers. High resistance method

Low resistance or Current zero interruption

SF6 Circuit Breaker :

PNEUMATIC mechanism is used

Short circuit earthing current is 40KA

Operating impulse with stand voltage is 1425KV

Operating sequence is 0 0.3 sec CO - 3 min CO

Air pressure is 31.5 bar

SF6 GAS pressure is 7.0 bar


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Isolators are disconnecting switches which can be

used for disconnecting a circuit under no currentcondition. They are generally installed along with the circuit breaker. A Isolator, can

be opened after the circuit breaker.

There are different types of isolators Now let us discuss about two types

of isolators which are used in this sub-station.


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Lightning arresters are equipments

which are connected between the

conductor and ground, to discharge the

excessive voltages to the ground.

It operates on the fact that a thin film of

aluminium hydroxide is deposited on the

aluminium plates immersed in electrolyte acts as

a high resistance to a low voltage but a low

resistance to a voltage above a critical value.


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Current transformers used in conjunction with

ammeter, over current relays, etc. which step downs the current

from high level to low value.

Core number 1 2 34 5

Core operation Bus-bar

protection

Spare one Used for

metering

Main II

distance

protection

Main I

distance

protection

Primary turns 1 1 1 1 1

Secondary turns 2000 1000 2000 1000 500

Rated primary

current 2000A 2000A 2000A 2000A 2000A

Rated

secondary

current

1A 1A 1A 1A 1A

Current output --- --- 40VA --- ---

Accuracy class PS PS 0.5 PS PS


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Primary terminal

Secondary terminal box

Core container

Expansion chamber

Oil level gauge

Porcelain insulator

Core


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Capacitor Voltage Transformers are used for

line voltmeters, synchroscopes, protective relays,

etc. which step downs the voltage from high level to

low value.

The capacitors connected in series act likepotential dividers provided the current taken by

the burden is negligible compared with the current

passing through the series connected capacitors.

Core 1: rating is 400kv/3/110/3 used for Main Idistance protection

Core 2: rating is 400kv/3/110/3 used for Main IIdistance protection

Core 3: rating is 400kv/3/110/3 used formetering.


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Type of

cooling

ONAN

Rated

power

63MVAR

Rated

voltage

420KV

Rated

current

82A

phases 3

frequency 50Hz

Impedance 2800Ohm/Ph

Reactor is an inductive reactance in comparisonto its resistance and is used for limiting short circuit during

fault conditions.

Reactors are used to reduce the magnitude ofvoltage disturbances caused by short circuits.


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The PLCC (Power line

carrier communication) equipment

contains wave-trap, tuning unit,coupling capacitor etc.

The PLCC is mainly

used for telecommunication, tele-

portection and tele-monitoring

between electrical substation

through power lines at high voltages

such as 110 kV, 220 kV, 400 kV.


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The supply available in nature is alternating which is

not stored in any form. If the AC supply failed then there is no

supply to relay coils .This needs availability of DC supply.

Boost charging:

When there is a sudden drop in the voltage/also when

there is considerable fall in voltage the charger will

automatically switch on to boost charging mode from float

charging mode.

Float charging:This is a constant charging mode means these

batteries will be constantly being charged when there is no

load it takes less current. And these cells have to be charged

continuously so that they will not get discharged. As these has

to supply DC supply when there is a fail in AC supply.


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In generating stations, receiving stations and sub-

stations, the control and relaying equipment is installed in

control-rooms.

The panels are divided on the bases of type of the operation it done they are:

Panel with Mimic Diagram.

Relay Panel. The type and number of relays depends on requirements.

Instrument pane. Indicating ammeter, voltmeter, energy meter, their

selector switches, recording instruments, if any, fitted on instrument panel.

Synchronizing panel.

Automatic voltage regulator panel


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The Transformer is the main equipment which has to be

protected mainly in the power system. They are different faults in it,

these areTHROUGH FAULTS

INTERNAL FAULTS

EXTERNAL FAULTS

INCIPIENT FAULTS

Incipient faults bellow oil level resulting

in decomposition of oil, faults between

phase to phase and ground.

1. Buchholz relay sounds alarm (Gas

actuated relay).

2. Percentage differential protection.

Saturation of magnetic circuit 1. Over fluxing protection

2.Over voltage protection

Earth faults Differential protection.

Though faults Graded time lag over current relay.

Overloads Thermal overload relays.

High voltage surges due to lightning 1. Surge arresters

2. R-C surge suppressors


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Buchholzs relay is gas actuated relay used for

protecting oil immersed transformer against all types of

internal faults.

Buchholz relay is provide for transformers above 500kVA rating.

Which is fitted in pipe leading to the conservator.

The minimum operating time of the equipment is 0.2sec.

Which is inclined in between 10

0

to 110

with the transformer tank. Mercury switch is immersed in it for closing alarm switch.

It gives alarm when oil level reduced to certain level due to

internal faults.


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The DIFFERENT IAL PROTECTION Is used for

power transformers is based on MERZ-PRICE circulating

current principle. It responds to vector difference between

to similar electrical quantities.

It provides protection against internal phase-to-phase and phase-to-

earth faults.

It generally generally used for transformers of rating exceeding 2MVA.

Mostly the operation is carried out by current difference.

The two basic requirements of differential relay connections must

satisfy these are..

It must not be operated for external or load faults

It must operate for internal faults


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R

Y

B

R

Y

BY

POWER TRANSFORMERC.T C.T

PILOT WIRES

TRIP

COIL

TRIP

COIL

KICK

FUSES

RELAY

OPERATING

COIL

For the transformer differential protection an ABB

make RADSB relay which is fully digitalized numerical relay.

This protective relay is intended for all types of autotransformers.


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Earth Fault Relay connected in residual circuit of

line CTs provide protection against the earth faults on the

delta or unearthed star connected windings of the power

transformers.

It provide in

parallel with thesecondaries of

CTs of protected

line.

Operating

current is 15%of rated winding

current.

For this

protection the

relay used is

RADHD

F-1F-2

I1

I2

Stabilising

resistor

CTs

Earth fault

relay

Neutral

CT


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Power transformers directly connected to alternators

subjected to over excitation which are during start up or shut

down operation.Over fluxing condition in a transformer can occur during

system over voltage and / or under frequency conditions (V/F)

For over fluxing protection the relay here used is ABB made

RALK.

When the power transformers are over excited

the leakage flux increases and this results in heavy hysteresis

and eddy current losses in non-laminated parts of

transformer. This leads to temperature raise and causes

insulation failure and hence flash over will occur.

The main causes for over fluxing are.


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The BUS-BAR in electric power stations and

substations are the one of the most vital elements. The

bus-bar protection needs special attention because of

the following reasons.

Faulty level at bus-bars is very high.

The fault on the bus-bars would result in widespread supply

interruption.

The system stability is adversely affected by fault in bus

zones.


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Method Particulars

Bus-protection by over current

relays, of connected circuits.

High-set instantaneous over current

relays and earth fault relays, or

definite time relays.

Static protection Rapid reliable, no problems of CT

structure

Back-up protection Over current protection or distance

protection.

Over voltage protection Inverse overvoltage relays

Surge voltage protection Surge arresters


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The principle of operation is the current entering

and leaving are compared continuously. It involves

choosing of impedence high enough to stabilize therelay for high fault currents.

For the busbar protection we are using ABB make RADSS which is

fully digitalized numerical relay

This is a high speed , sensitive moderately high impudence

differential relay.

Operating time for fault detection is 1-3 m.sec and for tripping 8-13

msec.


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The probability of faults occurrence on the

overhead lines is much more due to their greater lengths

and exposure to atmospheric conditions.

The transmission lines have the following protection schemes.

Distance protection

Main I distance protection

Main II distance protection

Over voltage protection

Stage I

Stage II

Auto reclosures

Breaker falure relay


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To detect excessive voltage rise on

transmission lines, usually 400Kv- 800Kv systems. The relay

would typically be time delayed to prevent unnecessary tripping

for voltage rise of short duration. The relay can also be used forgenerator over voltage protection.

For this protection an ABB make RXEG 21 is used

Main features:

Flash over protection. >= 97% for operation on over voltage.

High over load capacity.

Wide frequency response.


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RELAY is an electrical device designed to initiateisolation of part of an electrical installation, or to operate

an alarm signal, in the event of an abnormal condition or a

fault.

1. Circuit Breaker2. Relay

3. Trip coil of C.B

4. Trip circuit

5. Battery

6. Currenttransformer

7. Potential

transformer+ -


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When fault occurs CT or PT step-down the corresponding

parameter(I or V)

This quantity is passed to the corresponding relay.

Which produce the magnetic field to close the trip circuit.

CT

C.B

B+ -

R


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After closing of the trip switch closed trip circuit is formed.

Battery discharge the energy and magnetize the trip-coil

of the C.B.

Due to this circuit breaker is opened due to this supply isturned off.

C.B

B

CT

R


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There are basically three types of relays these are..

Static relays.

Electromagnetic relays.

Numerical relays.The Static Relay is a relay which incorporates solid statecomponent like transistors, diodes etc. for the measurement

of comparison of electrical quantities.

In Electromagnetic Relays, there are one or more coils,movable elements, contact system etc. The operation of

such relay depends on whether the operating torque/force

is greater than the restraining torque/force.


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Operation of the Impedance Relay is independent ofthe phase angle between V and I. The operating characteristic

is a circle with its center at the origin and hence the relay is

non-directional

Reactance relay measures V/I sin when ever thereactance measured by the relay is less than the set value

the relay operates. The operating characteristic on R-X

diagram.

Mho relay is a directional impedance relay,also known as admittance relay. Its characteristics on R-X

diagram is a circle whose circumference passes through

the origin. This relay is inherently directional and it only

operates for fault in the forward direction.


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Overview of Substation

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